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THE SETTING 11 from 5 to more than 15 miles. Its grandeur and impressiveness are due in large part to the sculpture of its sides. Throughout much of its length the canyon appears in cross section to have a flat bolttom of considerable width trenched by a steep-walled inner gorge; above thIs gorge, the almost hori- zontal geological formations retreat in terraces edged by cliffs from a few feet to a hundred feet high, except for the massive Redwall limestone, about halfway up, which stands out in a spectacular cliff several hundred to a thousand feet high. The Colorado River began its laborious cutting of the Grand Canyon several million years ago (Wilson, 1967, p. 61). Through the canyon, the river today maintains a grade 15 times as steep as that of the Ohio River. The details of its entrenchment are more visible near river level, where they have not been blurred by secondary erosion, than from the canyon rim some 5,000 feet above, where the river's total effort c~m be reviewed. In many places the lower ten feet of the inner gorge (vertical walls of reddish sandy limestone or of black metamorphk rock) shines like a mirror from continuous polishing by river silt over thousands of years and the sub- sequent development of the black patina called "'desert varnish." Beds of nearly pure limestone occur at a few spots in the inner canyon, particularly at the lower end of Marble Canyon, where the limestone surfaces are intricately fluted with upriver-facing cusps ranging in size from 1 to 20 inches. Such fluting of limestone by solution- and silt-polishing is rare. Although the tributary canyons carry streamflow infrequently and often only in small amounts, they meet the main stream at its own level. If each valley is the product of the stream that flows through it, these ephemeral side streams must have enormous energy in order to keep their canyon mouths at the level of the Colorado. On the othl~r hand, processes still not understood may facilitate downcutting of the tributary canyons. The fauna and flora of the Grand Canyon have some unique aspects. Redbud and maidenhair fern, plants unknown in most of the basin, grow in the narrow canyons where they are nourished by small springs and partly shielded from the intense sunshine. A large scorpion, more than four inches long, is found in the canyon. A curious fish, the humpbacked chub, whose only natural habitat is the canyon, appears to have adapted to the exceptional turbulence and velocity of the river there (Miller, 1946). The unique features of the Grand Canyon were recognized early by the American public. The Santa Fe Railroad made it a scenic attraction by constructing a track to the south rim in 1901. A reach of canyon and river 132 miles long was reserved for the public by the creation of Grand Canyon National Park in 1919 and Grand Canyon National Monument in 1932.